In order to assist the pedal force which is applied to a brake pedal of a vehicle for pressurizing a master cylinder, generally employed is a vacuum booster utilizing vacuum pressure generated in an intake manifold of an engine. On the other hand, a system of performing an antilocking control of a wheel brake for a vehicle through use of an auxiliary power source regularly storing high auxiliary dynamic pressure has come into wide use in recent years. It is known that a hydraulic booster is employed to utilize hydraulic pressure from the auxiliary power source in this case. Such a hydraulic booster can be easily reduced in size and increased in magnification as compared with the vacuum booster. Thus, the hydraulic booster can be integrated with a master cylinder and an antilocking control apparatus, to be easily mounted on a vehicle. For example, Japanese Patent Laying-Open Gazette No. 104449/1982 discloses a hydraulic booster which is integrated with an antilocking apparatus.
FIG. 9 is a sectional view showing a typical example of a hydraulic booster, which is integrated with a master cylinder.
A master cylinder 7 and a hydraulic booster 8 are integrated into a housing 1, which is provided with ports 2, 3, 4, 5 and 6. The port 2 is adapted to introduce fluid from a reserve tank 9 into a pressure chamber 10 of the master cylinder 7, and the port 3 is adapted to discharge the fluid from the pressure chamber 10 toward a front wheel brake 11. The port 4 is adapted to discharge fluid exhausted from the hydraulic booster 8 to the reserve tank 9 through a chamber 13 which is provided on an outer peripheral portion of a master piston 12 of the master cylinder 7. The port 5 is adapted to guide auxiliary dynamic pressure from an auxiliary power source 14 to the booster 8, while the port 6 is adapted to guide boost pressure formed in a boost chamber 15 of the booster 8 to a rear wheel brake 16.
The booster 8 comprises an input member 17, a spool 18 and a boost piston 19. The input member 17 is coupled to an input rod 20 which is driven by a brake pedal, and the spool 18 is coupled to the input member 17. The input member 17 and the spool 18 are relatively movable with respect to the boost piston 19.
The boost piston 19 is provided with a communication hole 21 which extends from its outer peripheral portion to a central opening, an input hole 22, a communication hole 23 and an output hole 24. The spool 18 is provided with a central communication hole 25 which is formed in its central portion and communication holes 26 and 27 connecting its outer peripheral portion with the central communication hole 25. Further, a small groove 28 is defined on the outer peripheral portion of the spool 18.
Two seals 29 and 30 are provided to hold the communication hole 23 and the output hole 24 of the boost piston 19 between the same. The boost chamber 15 is formed in a stepped portion of the boost piston 19, which is sealed by the two seals 29 and 30.
As shown in FIG. 9, further, a plunger 32 having a communication hole 31 in its interior is contained in the central opening of the boost piston 19.
In the state shown in FIG. 9, the input hole 22 of the boost piston 19 is closed by the outer peripheral surface of the spool 18, whereby the auxiliary dynamic pressure transmitted from the auxiliary power source 14 through the port 5, the outer peripheral portion of the boost piston 19 and the input hole 22 is interrupted by the spool 18. Further, the boost chamber 15, which is connected with the rear wheel brake 16 through the port 6, communicates with the reserve tank 9 through the communication hole 26 and the central communication hole 25 of the spool 18, the communication hole 31 of the plunger 32, the communication hole 21 of the boost piston 19 and the chamber 13 provided on the outer peripheral portion of the master piston 12.
The master piston 12 is coupled to the boost piston 19, with a reaction disc 33 being interposed between the same. In the housing 1, a holddown member 34 is fixed in a position opposite to the master piston 12 by the pressure chamber 10. The holddown member 34 contains in its central opening a filter 35 which faces the port 2 communicating with the reserve tank 9 and a valve seat 36.
A rod 38 having a ball valve 37 for closing the valve seat 36 is movably contained in the pressure chamber 10. The rod 38 is urged by a spring 39 toward the valve seat 36. In the state shown in FIG. 9, the rod 38 is engaged with a cap 40 which is mounted on an end portion of the master piston 12 for preventing the rod 38 to move to the left in FIG. 9. In this state, the ball valve 37 is separated from the valve seat 36. Thus, the pressure chamber 10 of the master cylinder 7 communicates with the reserve tank 9 in the state shown in FIG. 9. Further, a master spring 41 is provided between the holddown member 34 and the master piston 12, to separate the master piston 12 from the holddown member 34.
In the state shown in FIG. 9, no pedal force is applied through the brake pedal. In this state, the boost piston 19 is rearwardly, i.e., rightwardly urged by the master spring 41 and auxiliary dynamic pressure from the auxiliary power source 14, which acts on an effective sectional area difference between the seals 29 and 29' (29&gt;29'). A shoulder portion 43 of the boost piston 19 facing the boost chamber 15 comes into contact with a seal holder 42 which is fixed to the housing 1, thereby to stop such rearward movement of the boost piston 19.
A C-shaped snap ring 44 is fixedly mounted on a rear end portion of the boost piston 19. The rear end portion of the input member 17 comes into contact with the C-shaped snap ring 44, thereby to define the terminating end of its rearward movement.
The hydraulic booster 8, which is integrated with the master cylinder 7, operates as follows.
When a braking operation is started and the input rod 20 coupled to the brake pedal, moves to a pressurizing side, the spool 18 is brought into contact with the plunger 32, to compress and deform the reaction disc 33, which is made of rubber. At this time, communication between the output hole 24 of the boost piston 19 and the communication hole 26 of the spool 18 is cut off. The input hole 22 of the boost piston 19 communicates with the small groove 28 of the spool 18, whereby the auxiliary dynamic pressure from the auxiliary power source 14 is introduced into the boost chamber 15 through the port 5, the input hole 22, the small groove 28 and the communication hole 23. Then the boost piston 19 is driven in the forward direction by boost pressure acting on an area difference between sealing portions of the seals 29 and 30, to compress the reaction disc 33.
The reaction force of the disc 33 thus compressed is increased by the mechanism of a reaction disc which is well known in relation to a vacuum booster, and such increased reaction force is transmitted to the spool 18 through the plunger 32. Consequently, the spool 18 returns to its original position, whereby the communication between the input hole 22 of the boost piston 19 and the small groove 28 of the spool 18 is again cut off. At this time, the communication hole 26 of the spool 18 does not communicate with the output hole 24 of the boost piston 19. Consequently, boost pressure which is responsive to the thrust of the input rod 20 is generated in the boost chamber 15, to push the master piston 12.
When the master piston 12 thus moves to the left in FIG. 9, the rod 38 having the valve 37 is driven by the spring 39 to close the valve seat 36. When the thrust of the input rod 20 is increased in this state, the master cylinder 7 is pressurized by a repetition of the aforementioned operation.
When no pedal force is applied to the input rod 20 in the apparatus shown in FIG. 9, the boost pressure must be reduced to zero to bring the boost piston 19 into contact with the seal holder 42 by closing an input directional control valve for allowing or cutting off communication between the auxiliary power source 14 and the boost chamber 15 while opening an output directional control valve for allowing or cutting off communication between the boost chamber 15 and the reserve tank 9. In the known apparatus shown in FIG. 9, spool type input and output directional control valves are defined by the input hole 22 provided in the boost piston 19, the introduction hole 23 to the boost chamber 15, the output hole 24 from the boost chamber 15 and the spool 18.
When the input member 17 and the boost piston 19 rest in the rearmost positions as shown in FIG. 9, an input edge 45 of the spool 18 is lapped over the input hole 22 by l.sub.1, thereby to cut off communication through the input hole 22, the small groove 28, the introduction hole 23 and the boost chamber 15. On the other hand, an output edge 46 of the spool 18 and the output hole 24 are opened by l.sub.2, to allow communication through the boost chamber 15, the output hole 24, the communication hole 26, the central communication hole 25, the communication hole 31, the chamber 13 and the reserve tank 9. The boost pressure in the boost chamber 15 is zero in this case.
Upon starting a braking operation in the aforementioned conventional hydraulic booster, a travel loss is caused before the hydraulic booster starts its operation, which causes an undesirable pedal feeling. Namely, when pedal force is applied to the input rod 20, the input member 17 starts to move forward with the spool 18. When the spool 18 moves forward toward the front end by the stroke of l.sub.2, the output directional control valve defined by the output edge 46 of the spool 18 and the output hole 24 is closed. When the spool 18 further moves forward still by the stroke of l.sub.1 -l.sub.2, the input directional control valve defined by the input edge 45 of the spool 18 and the input hole 22, is opened. Thus, travel loss of the length l.sub.1 is caused before the booster 8 starts its normal operation. Consequently, a pedal travel is wasted by the product of the travel loss and a pedal ratio, to cause a spongy pedal feeling which is undesirable.
It is extremely difficult to reduce the lap length l.sub.1 and l.sub.2, in view of a needed working accuracy. Namely, the length l.sub.1 of the lap portion between the input edges 45 and the input hole 22 must be larger than the length l.sub.2 of the lap portion between the output hole 24 and the output edge 46 in the first place. If the length l.sub.1 is reduced while maintaining such relation, leakage of auxiliary hydraulic pressure is increased in a non-braking state. Thus, the more often such an auxiliary hydraulic pressure generator is operated, the more energy is wasted. If the length l.sub.2 is reduced, further, a delay is caused in the pressure reduction due to an area reduction of the fluid passages. In the apparatus shown in FIG. 9, therefore, the travel loss in an initial stage of a braking action by the hydraulic booster cannot be reduced.
The aforementioned problem is not specific to the hydraulic booster shown in FIG. 9. A similar problem is caused in a hydraulic booster which positions the input member 17 rearwardly by the boost piston 19 while moving back the boost piston 19 by the housing 1, since it is necessary to reliably close the input directional control valve while opening the output directional control valve in an inactive state when a pedal force is not applied. For example, the aforementioned problem is caused in an apparatus having pressure regulating means separated from the body of a boost piston by a lever, which is disclosed in Japanese Patent Laying-Open Gazette No. 145655/1984. Further, a similar problem is also caused in an apparatus employing a concentric valve member formed by concentrically arranging input and output directional control valves similar to those of a conventional vacuum booster in place of a spool valve, which is disclosed in SAE Technical Paper Series 840465, Society of Automotive Engineers, Inc., U.S.A.